1. Field of the Invention
This invention concerns reproduction equipment for AV. In the present specification, the term “AV” signals refers to either audio or video or both audio and video (the form in which these coexist is arbitrary). To be more specific, the invention concerns an art of embedding an identification code, etc., which specifies a reproduction equipment, as a digital watermark in AV signals (reproduced signals) output from the reproduction equipment.
This invention also concerns equipment which specifies the reproduction equipment, which extracts a digital watermark from reproduced AV signals, and which specifies the reproduction equipment that produced the AV signals (reproduced signals).
In the present Specification, “reproduction equipment” refers not just to equipment that have only the function of reproducing AV signals from a recording medium but refers inclusively to equipment having both the function of reproduction and the function of recording AV signals in a recording medium.
2. Description of the Related Art
Recently, it has become possible to record and reproduce AV data in various forms. AV data can be recorded and reproduced as analog signals, as in the case of VHS, or recorded and reproduced as digital signals, as in the case of DVD.
Also, the AV data may consist of just audio signals or video (moving pictures or still pictures) signals and can be recorded and reproduced as analog signals or digital signals.
This invention can be applied to AV data of the various abovementioned signal forms.
In particular, digital contents (AV data in the form of digital signals) have come to be used widely recently.
Regardless of whether the data that are recorded and reproduced as AV data take on an analog or digital form, if the data have the property of being an authored work, the copyright thereof must be protected. The prohibition of illegal copies is especially important.
Although AV data that take the form of analog signals can be copied, the degradation of quality (sound quality or picture quality, etc.), with each generation, cannot be avoided when copying is performed. It can thus be said that excessive copying of analog AV data cannot be performed readily.
However, quality degradation from copying does not occur at all with digital contents. In other words, a copy that is exactly the same as the original is easy to make.
Since illegal copies of contents (or contents that originate from such contents and are distributed) cannot be distinguished from the original, it is difficult to provide evidence that proves their illegality. The establishment of technical methods that enable effective protection of copyrights is thus being demanded.
The use of “digital watermarks” is being examined as such a method.
A “digital watermark” consists of data that are embedded in AV data in a manner that is imperceptible to a viewer or listener and are devised so that the digital watermark can be extracted from the AV data in which it is embedded.
Copyright information (for example, the name of the copyright holder, date of copyright, etc.) are embedded in AV data as a watermark for protection of the copyright.
By extracting the embedded digital watermark (copyright information) from AV data that are suspected to have been copied illegally, the copyright holder, etc. of the contents are clarified and the illegality of a copy is exposed. In such a manner, digital watermarks are intended to prevent illegal copies.
Specific techniques for embedding a watermark in AV data are described in detail, for example in “The Fundamentals of Digital Watermarks—A New Protection Technology for Multimedia” by Kashio Matsui (Morikita Publishing, 1998), etc.
Japanese patent publication No. 2982768 also discloses the prevention of illegal copying of contents by embedding a digital watermark in AV signals.
In the art described by this literature, an illegal copy prevention control signal is embedded in advance as a digital watermark in the AV signals.
Extraction of the illegal copy prevention control signal from the AV signals is then performed during the reproduction of the AV signals. If the copy prevention signal has been extracted turns out to be illegal, noise is added anew as a digital watermark.
Thus each time an illegal copy of a recording medium, prepared based on the above art is viewed or listened to, noise is added to the AV signals. This degrades the quality (sound quality of visual quality) of the AV signals each time the signal is reproduced or copied. Eventually, such a large amount of noise is added to the AV signals that viewing and listening is impossible. As a result, illegal copying in multiple generations is prevented.